This invention relates to fabric components used in personal absorbent articles such as diapers and feminine hygiene products, which are worn against the body of the user and are employed to absorb bodily liquids and exudates.
Disposable articles of this nature generally include an outer waterproof layer of film, an inner absorbent core, and a cover sheet made of nonwoven fabric. The absorbent core generally comprises cellulose pulp, and a variety of types of nonwovens have been used as the cover sheet, especially calendered or point bonded polyolefin webs of fibers. To provide requisite softness, the cover sheet typically comprises fibers in the range of 1.7 to 3.3 decitex. A smaller fiber would not provide adequate porosity in the fabric, and a fabric made up of a larger fiber would feel harsh against the skin.
Since the mid-1980's, superabsorbent polymers (SAP) have been incorporated into cellulose diaper cores. These polymers are capable of absorbing up to 40 times their weight in body liquids and are more cost effective than fluff pulp alone. The SAP particles are either mixed with the pulp or are incorporated by layering, and the structure is usually densified to reduce bulk.
With the advent of the use of SAP in absorbent cores, other problems have arisen. Dense core structures generally require a longer time to absorb liquids, and the SAP particles may swell and form gel blocks which impede the liquid acquisition rate of the core. A slow acquisition or absorption rate by the core may result in runoff and leakage of excess liquids.
In order to deal with the above problem, liquid acquisition layers have been employed, namely, a relatively porous and sometimes bulky layer of nonwoven fabric inserted between the cover sheet and the porous core. The pore size of the transfer layer is usually larger than the pore size of the cover sheet. The acquisition layer provides additional void volume for liquids at the interface into the core, and generally provides a more uniform distribution of liquids. This allows the cover sheet of the diaper to feel dry while the core is absorbing the liquid.
Nonwoven fabrics used in acquisition layers are made by a variety of methods, including thermal point bonding of polymer fibers, power bonding and through-air bonding. Adhesive bonding is also employed wherein a web of fibers is passed through or sprayed with an adhesive emulsion, and the emulsion is cured or set by heating. These cured adhesives are characterized by having a glass transition temperature, or Tg, at or above which the polymer chains are mobile or can freely rotate. Adhesives previously used in acquisition layers have had a Tg in the order of -5.degree. C. to 20.degree. C.
One important property of liquid acquisition layers is the ability to retain their compressive resiliency in both the dry and wet states. If the porous structure of the fabric collapses, the void volume is greatly reduced, and the fabric can no longer function as a transfer layer. For example, if the transfer layer suffers loss of resilience, void volume or porosity after an initial liquid insult, subsequent insults will not be accommodated, and the article will leak, even though there remains sufficient liquid capacity in the core to eventually absorb the excess liquid.